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Message-ID: <CAP22eLHres_shVWEC+2=wcKXRsQzfNKDAnyRd8yuO_gJ3Wi_JA@mail.gmail.com>
Date: Wed, 6 May 2020 16:58:16 +0300
From: "Lev R. Oshvang ." <levonshe@...il.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
Alexei Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
Andy Lutomirski <luto@...nel.org>, Christian Heimes <christian@...hon.org>,
Daniel Borkmann <daniel@...earbox.net>, Deven Bowers <deven.desai@...ux.microsoft.com>,
Eric Chiang <ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>,
James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>,
Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>, Matthew Garrett <mjg59@...gle.com>,
Matthew Wilcox <willy@...radead.org>, Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
Mimi Zohar <zohar@...ux.ibm.com>,
Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
Scott Shell <scottsh@...rosoft.com>,
Sean Christopherson <sean.j.christopherson@...el.com>, Shuah Khan <shuah@...nel.org>,
Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-integrity@...r.kernel.org,
LSM List <linux-security-module@...r.kernel.org>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 0/6] Add support for O_MAYEXEC
On Tue, May 5, 2020 at 6:36 PM Mickaël Salaün <mic@...ikod.net> wrote:
>
>
> On 05/05/2020 17:31, Mickaël Salaün wrote:
> > Hi,
> >
> > This fifth patch series add new kernel configurations (OMAYEXEC_STATIC,
> > OMAYEXEC_ENFORCE_MOUNT, and OMAYEXEC_ENFORCE_FILE) to enable to
> > configure the security policy at kernel build time. As requested by
> > Mimi Zohar, I completed the series with one of her patches for IMA.
> >
> > The goal of this patch series is to enable to control script execution
> > with interpreters help. A new O_MAYEXEC flag, usable through
> > openat2(2), is added to enable userspace script interpreter to delegate
> > to the kernel (and thus the system security policy) the permission to
> > interpret/execute scripts or other files containing what can be seen as
> > commands.
> >
> > A simple system-wide security policy can be enforced by the system
> > administrator through a sysctl configuration consistent with the mount
> > points or the file access rights. The documentation patch explains the
> > prerequisites.
> >
> > Furthermore, the security policy can also be delegated to an LSM, either
> > a MAC system or an integrity system. For instance, the new kernel
> > MAY_OPENEXEC flag closes a major IMA measurement/appraisal interpreter
> > integrity gap by bringing the ability to check the use of scripts [1].
> > Other uses are expected, such as for openat2(2) [2], SGX integration
> > [3], bpffs [4] or IPE [5].
> >
> > Userspace needs to adapt to take advantage of this new feature. For
> > example, the PEP 578 [6] (Runtime Audit Hooks) enables Python 3.8 to be
> > extended with policy enforcement points related to code interpretation,
> > which can be used to align with the PowerShell audit features.
> > Additional Python security improvements (e.g. a limited interpreter
> > withou -c, stdin piping of code) are on their way.
> >
> > The initial idea come from CLIP OS 4 and the original implementation has
> > been used for more than 12 years:
> > https://github.com/clipos-archive/clipos4_doc
> >
> > An introduction to O_MAYEXEC was given at the Linux Security Summit
> > Europe 2018 - Linux Kernel Security Contributions by ANSSI:
> > https://www.youtube.com/watch?v=chNjCRtPKQY&t=17m15s
> > The "write xor execute" principle was explained at Kernel Recipes 2018 -
> > CLIP OS: a defense-in-depth OS:
> > https://www.youtube.com/watch?v=PjRE0uBtkHU&t=11m14s
> >
> > This patch series can be applied on top of v5.7-rc4. This can be tested
> > with CONFIG_SYSCTL. I would really appreciate constructive comments on
> > this patch series.
> >
> > Previous version:
> > https://lore.kernel.org/lkml/20200428175129.634352-1-mic@digikod.net/
>
> The previous version (v4) is
> https://lore.kernel.org/lkml/20200430132320.699508-1-mic@digikod.net/
Hi Michael
I have couple of question
1. Why you did not add O_MAYEXEC to open()?
Some time ago (around v4.14) open() did not return EINVAL when
VALID_OPEN_FLAGS check failed.
Now it does, so I do not see a problem that interpreter will use
simple open(), ( Although that path might be manipulated, but file
contents will be verified by IMA)
2. When you apply a new flag to mount, it means that IMA will check
all files under this mount and it does not matter whether the file in
question is a script or not.
IMHO it is too hard overhead for performance reasons.
Regards,
LEv
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